Lighting devices using LEDs or OLEDs (Organic Light Emitting Diode) gain more and more interest for general lighting applications. LEDs and OLEDs can be used to produce large amounts of light and have the benefit that they allow fast switching. On the one hand they can be used as general light sources, on the other hand they can be used as displays or design elements. In order to control the LEDs/OLEDs in a lighting device, so-called driver devices are used. In the art, there are a couple of solutions with respect to how to design such driver devices. For example, US 2006/0038803 A1 or WO 2006/107199 A2 disclose approaches regarding how to control LEDs coupled in series to form an LED string.
Generally, using a scanning matrix is the most obvious way to control a plurality of LEDs (or OLEDs) individually. However, the drawback is the low utilization of the individual LEDs. Due to multiplexing, only a fraction of the time cycle is used as activation time for each LED. Thus, the optical output of the LEDs will be lower than their nominal value. It is not possible to compensate for the dark time in larger setups, because in high power LEDs, the peak current is limited to a certain value.
In cases where only one string (i.e. a couple of LEDs arranged in one dimension, for example as a column or a row) is controlled, two possibilities are proposed in the art:
First, all LEDs can be connected to a common potential at one terminal, and the other terminal is switched. In this case, some current limiting means are necessary for each individual LED.
Second, it is possible to connect the LEDs in series. Here, only one current limiting block is required, but it is complicated to switch the individual LEDs due to their floating reference potential within the string.